The price of a security reflects the average market view of informed investors. Nevertheless, it is unclear to what extent investors actually use the information contained within the price when making decisions. DIW Berlin researched this question, in cooperation with the Humboldt University of Berlin, using a decision-making experiment. For this purpose, the decisions of investors in order-driven markets were mapped out. In such markets, investors place their orders before the stock price has been determined. The results were then compared with those that arose from a different situation in which investors first observe the price of a financial product, and then place an order. It turns out that the observability of the price influences the rationality of investment decisions. Investors make better use of the information about the expectations of other investors that is contained in the price if they observe the actual price.